1. Field of the Invention
The present invention generally relates to the control of electrostatic discharge, and more particularly to a device which monitors work surfaces and human operators to insure substantial elimination of electrostatic discharge in the manufacture or assembly of sensitive electronic components.
2. Description of the Prior Art
Electrostatic discharge, as well as the mere presence of a static electric field, can be extremely detrimental to sensitive electronic parts. This is particularly true of modern semiconductors and integrated circuits which may be degraded or destroyed by the buildup of static electricity in the workplace. Especially sensitive components can be severely affected by a potential as small as 50 volts, yet the simple act of walking has been known to triboelectrically generate a potential of 30,000 volts or more.
The most common tool heretofore used in the battle against electrostatic discharge is a conductive wrist strap which connects the wearer to a ground via a grounding tether. A wide variety of wrist straps have been devised, such as that described in U.S. Pat. No. 4,398,277 issued to Christiansen et al. The next logical step in improving upon this idea was to periodically check the conductive integrity of these wrist straps. For example, Minnesota Mining & Manufacturing Co. (assignee of the present invention) markets a wrist strap tester under the brand name "CHARGE-GUARD." This device not only test the maximum resistance of the strap (to insure adequate drainage of static buildup), but also checks the minimum resistance to insure that the user will be protected from dangerous levels of current which may be introduced in the path to ground.
A further refinement of the strap tester has resulted in devices which continually monitor the integrity of the strap. Two such devices are disclosed in U.S. Pat. No. 4,558,309 issued to J. Antonevich and U.S. Pat. No. 4,638,399 issued to Maroney et al. The Antonevich monitor constantly checks the electrical continuity between the wrist strap and ground, alerting the user when a break in continuity occurs. It does not, however, test for minimum resistance (i.e., a possible dangerous current path through the strap to ground). The Maroney et al. apparatus employs phase comparison between a control signal and a composite signal (affected by the strap integrity) to monitor the path to ground. It also does not check for a minimum resistance, although it does detect a short and also has a current limiting resistor in place to minimize shock hazard.
One particular disadvantage of the Maroney et al. and Antonevich devices relates to their use of the effective impedance of the individual wearing the wrist strap in evaluating the integrity of the strap and tether. The effective impedance is a function of both the resistance and capacitance of the individual. Therefore, it is possible to have a situation wherein the resistance is extremely high, but the monitor nevertheless detects a proper grounding resistance exists. For example, if an insulative or dielectric material is placed between the wrist strap and the skin of the individual, the ground path resistance becomes unacceptable, yet the capacitive coupling of the wrist strap with the skin can mislead the monitor into believing that a conductive path exists between the strap and the wearer. This problem is especially acute when the wearer has very dry skin, or has placed an insulative garment between the strap and skin.
While Maroney et al. and Antonevich utilize an AC signal (which is affected by the impedance of the person being monitored), some devices use a DC signal applied to the wriststrap. One such device is disclosed in U.S. Pat. No. 4,639,825 issued to A. Breidegam. This patent describes a monitoring circuit, as well as a special dual conductor wrist strap. The circuit constantly monitors the ground path, and is affected by the resistance across the wrist strap, not the effective impedance of the individual.
One disadvantage of continual monitors (particularly the DC monitors such as Breidegam) relates to the voltage impressed upon the surface of the skin of the person wearing the wrist strap (see, e.g., Maroney et al, col. 5, lines 24-38). Some individuals are particularly sensitive to electrical currents across the skin, and use of constant monitoring device can cause rashes and other skin eruptions to appear on such individuals. This effect is pronounced when the strap is worn day after day, which is typical for assembly line workers. Prior to the conception of the subject invention, no one has been able to provide continual monitoring of wrist straps without also subjecting workers to voltages which, over long periods, may result in great discomfort. Avoidance of constant electrical output provides an additional advantage when the power supply is portable, i.e., a battery.
The above-described devices are further limited in that they assist only in eliminating static buildup on the human technician, and do nothing to prevent electrostatic discharge emanating from the work surface. In this regard, static dissipative work surfaces, floor mats, etc., are presently used at work stations in order to prevent transient electromagnetic fields from affecting nearby electronic components. The static dissipative work surfaces, like the wrist straps, are also connected to ground to allow drainage of excess electrical charge.
One apparatus designed to monitor the grounding ability of work surfaces, as well as the technician's wrist strap, is illustrated in U.S. Pat. No. 4,649,374 issued to J. Hoigaard. This monitor, which probably represents the closest prior art with respect to the subject invention, also provides for detection of unsafe electrical potentials at the tip of a soldering iron. The Hoigaard device, however, still suffers from several disadvantages. First of all, the wrist strap is not continually monitored, but rather requires "touch-to-test" activation. This means that the user must remember to periodically check the strap during the day, i.e., "after starting work, each break, and after lunch" (col. 5, lines 46-53). Manual interaction with the device introduces obvious quality assurance problems. Furthermore, the visual indicators on the Hoigaard device are difficult to interpret as there is only a single lamp to indicate whether the strap integrity is acceptable. This creates a further problem if the lamp is burned out since, in such a case, there is no way to determine if the lamp is broken or the wrist strap grounding path is improper. It would, therefore, be desirable and advantageous to devise a wrist strap/work surface monitor which continually checks the integrity of the strap ground without irritation of the skin, and which also provides unambiguous visual and aural signals to the user.
Accordingly, the primary object of the present invention is to provide a monitor for checking the integrity of the grounding path for a conductive wrist strap.
Another object of the invention is to provide such a device which also monitors the dissipative characteristics of the work surface.
Still another object of the invention is to provide a wrist strap monitor which continually checks the strap grounding path without aggravating the skin of the user.
Yet another object of the invention is to provide a monitor having clear visual and aural indicators for specifying the status of the wrist strap and work surface ground paths.